Don't chop up anything just yet...take a look at this connector on eBay:
http://cgi.ebay.com/Motorola-MX-KVL-...QQcmdZViewItem
It's a Hirose adapter for the MX300 series, model number: PLN-6980A. The DEVGRU should plug directly into that. I think it's worth a shot, especially for the price, before you go to the trouble of ripping apart a speaker mic and wiring it up; I would imagine that if your DEVGRU works with a Saber or any other Motorola for which a Hirose adapter is available (Genesis series HT600, Jedi series HT1000, XTS3000, etc), then it should definitely work with your MX300R.
Would you mind sharing with me where you got your UHF MX300R, and which frequencies it came programmed to? You mentioned that you had more than one I believe in the original post; would you care to sell one to me? =) All the ones currently on eBay have been demilitarized (channel elements/PROM ripped). I'm having trouble finding a working one.
Here's a pic of my current setup, complete with hirose adapter:

Props for your excellent choice of radios! Thanks.